I was having the same problem… When the user try to vote at a photo that he already had voted, the error occurs.
So I included a error message at viewimg.php (line 796):
<div class=”feedback hidden” id=”error_captcha”><?php _e(‘Erro:’, ‘wp-photocontest’);?> <?php _e(‘Você j´ votou nessa foto.’, ‘wp-photocontest’);?></div>
